FIG. 1 shows a typical optical lithographical fabrication system 200, as used for delineating features in a wafer (substrate) 100, typically a semiconductor wafer. More specifically, optical radiation from an optical source 106, such as a mercury lamp, propagates through an aperture in an opaque screen 105, an optical collimating lens 104, a mask or reticle 103, and an optical focusing lens 102. The optical radiation emanating from the reticle 103 focused by the lens 102 on a photoresist layer 101 located on the top major surface of the wafer 100 or, more usually, on various layers on the top surface of the wafer 100. Thus, the pattern of the reticle 103--that is, its pattern of transparent and opaque portions--is focused on the photoresist layer 101. Depending upon whether this photoresist is positive or negative, when it is subjected to a developing process the material of the photoresist is removed or remains at and only at areas where the optical radiation was incident. Thus, the pattern of the mask is transferred to (printed on) the photoresist. Subsequent etching processes, such as wet etching or dry plasma etching, remove selected portions of the substrate or of layer(s) of material(s) (not shown) located between the top surface of the wafer and the bottom surface of the photoresist layer, or of both the substrate and the layer(s). Portions of the substrate or of the layer(s) of material thus are removed from the top surface of the wafer 100 underlying areas where the photoresist was removed by the developing process but not underlying areas where the photoresist remains. Thus, the pattern of the mask 103 is transferred to layer(s) of material(s) overlying the wafer 100, as is desired, for example, in the art of semiconductor integrated circuit fabrication.
In fabricating such circuits, it is desirable to have as many devices, such as transistors, per wafer. Hence it is desirable to have as small a transistor or other feature size as possible, such as the feature size of a metallization stripe--i.e., its width W--or of an isolated aperture in an insulating layer which is to be filled with metal, in order to form electrical connections, for example, between one level of metallization and another. Thus, if it is desired to print the corresponding isolated feature having a width equal to W on the photoresist layer 101, there must exist a feature having a width equal to C located on the mask (reticle) 103. According to geometric optics, if this feature of width C is a simple aperture in an opaque layer, then the ratio W/C=m, where m=L2/L1, i.e., the image distance divided by the object distance, and where m is known as the lateral magnification. When diffraction effects become important, however, the edges of the image become fuzzy and lose their sharpness; and hence the so-called resolution of the mask features when focused on the photoresist layer 101 deteriorates.
In a paper entitled "New Phase-Shifting Mask with Self-Aligned Phase Shifters for a Quarter Micron Lithography" published in International Electron Device Meeting (IEDM) Technical Digest, pp. 57-60 (3.3.1-3.3.4) (December, 1989), A. Nitayama et al. taught the use of masks having such features as isolated apertures transparent phase-shifting portions to achieve improved resolution--i.e., improved sharpness of the image of the mask features when focused on the photoresist layer 101. More specifically, these masks comprised suitably patterned transparent optical phase-shifting layers, i.e., layers having edges located at predetermined distances from the edges of the opaque portions of the mask. Each of these phase-shifting layers had a thickness t equal to .lambda./2(n-1), where .lambda. is the wavelength of the optical radiation from the source 106 (FIG. 1) and n is the refractive index of the phase-shifting layers, and thus these layers introduced phase shifts (delays) of .pi. radians in the optical radiation. By virtue of diffraction principles, the presence of these phase-shifting layers in the masks assertedly produces the desired improved resolution. Such masks are called " phase-shifting " masks.
The mask structure described by A. Nitayama, op.cit., was manufactured by a process involving a step of wet etching (laterally) an opaque chromium layer located underneath a phase-shifting layer of PMMA which is resistant to the wet etching, whereby the etching of the chromium layer undercut the PMMA layer and formed a phase-shifting mask. However, the amount of chromium thus etched, and hence the positioning of its edges, is difficult to control. Yet the positioning of the edges of the opaque chromium must be carefully controlled in order to yield the desired improved resolution for the mask.
A two-resist-level, non-self-aligned method for manufacturing phase-shifting masks is disclosed in a patent application (J. G. Garofalo 3-4-13) U.S. Pat. No. 07/622680 entitled "Phase-Shifting Lithographic Masks with Improved Resolution" filed concurrently herewith on behalf of the same inventors as those of the present application. However, it would be desirable to have a more alignment-tolerant method of making such masks.